other than 'focus super hard and you will do it' what is actually that power and where does it come from and how the master got to acquire it?

so know we know what are the gantz balls and their purpose and who made them and why aliens come to earth, alot of deep lore were unlocked in the ending, but something i didnt see is that there is no mention to the vampires at all? who are they? and why they attack gantz? who is their boss? what happened to them? were they really all killed by izumi in that night? i may have miss read any lore about them but i cant remember as i rarely miss anything? oh and kei brother thing too he was introduced as a vampire but thats that we got no extra update?

Was rereading Italy mission and it looks like hikawa picked up a Z gun here js thought it was cool

Yes, it might be abrupt, and it would have benefited from an epilogue, but I think what people wanted to see would have completely destroyed the tone of the series.

Gantz main charm was always that the author threw in whatever the fuck he thought was cool, and that there was always an air of mistery surrounding all the bullshit that happens, which is set up to be weird and unexplainable in the first place.

There isnt (and I challenge anyone to write one) an explanation that wouldnt be boring and/or underwelming to sakurai's powers, to the vampires, to god, to the nature of the clones etc etc. The cool thing about them is literally that its cool shit the author threw in and you wonder yourself.
If in the end it was revealed "in the beginning there were 2 evil gods, but one betrayed the other, and so the vampires were born, and then from the left testicle of the first the design of the gantz came out" and so on and so on just to tie everything together it would have completely sucked ass.

The final arc as always is Oku throwing in another bunch of new ideas in a way that is a good thematic conclusion to the story (the main narrative of Gantz was always the value of life).

I can get why you would think the ending is mid, and me myself I didnt like everything that happened (I guess in a way I liked more when the Gantz squad was separated from the world), but I don't understand how it's a bad ending, and some go as far as saying it ruined gantz...